The isolation medium for Abiotrophia, the isolation ratio of Abiotrophia, from saliva and dental plaque of healthy adults, production of dextran and protease, and antimicrobial susceptibility of Abiotrophia were studied. The most appropriate medium for isolation of Abiotrophia was a double-layer agar plate made with a base layer of Columbia agar and a top layer of Columbia agar containing heat-killed Micrococcus luteus to reach the optical density of 2.O units, pyridoxal and cysteine. Samples were taken from 93 healthy young human. Out of these human subjects, 72 strains of Abiotrophia (77.4%) were isolated from saliva and dental plaque each. No isolation was observed in three human subjects. This suggests that Abiotrophia consists of normal flora in oral cavity. When 423 isolated Abiotrophia strains were identified, 192 of the 212 strains from saliva were A. adiacens (90.6%), and 20 (9.4%) were A. defectiva. Of the 211 strains from dental plaque, 180 were A. adiacens (85.3%) and 31 (14.7%) were A. defectiva. All of the strains produced dextran, and 28.7% of of A. adiacens and 66.7% of A. defectiva produced protease. By the examination of the antimicrobial susceptibilities of 22 agents of the 92 strains of Abiotrophia. both A. adiacens and A, defectiva were high susceptible to antimicrobial agents tested: benzylpenicillin, ampicillin, amoxicillin, aspoxicillin, cephalexin, cefazolin, cefotaxime, cefpirome, flomoxef, erythromycin, leucomycin and clindamycin. To imipenem/cilastain, streptomycin, gentamicin, lincomycin and vancomycin, A. adiacens were lower than A. defectiva. Minimum bactericidal concentration/minimum inhibitory concentration ratio ≧ 2 occurred in benzylpenicillin. This indicates no penicillin tolerant strains were found.